Optical interferometry has been widely used for accurate measurement of various physical, chemical and biological quantities. Optical interference superposes two or more coherent optical waves of certain propagation delays to generate periodic patterns in time, space, or frequency domain. The information embedded in the periodic patterns such as the phase, the amplitude, and the frequency positions of the waves can be utilized to compute the propagation delays. An interferometer can be designed to encode the information to be measured into the propagation delays. Thus, an interferometric sensor can be used to measure various parameters. Optical interferometric sensors and measurement techniques have high sensitivity, high response frequency, immunity to electromagnetic interference (EMI), remote operation, low optical attenuation and the ability to be transmitted over the long distance.
The principle has been implemented into various sensors and instruments. Based on the different ways of generating, separating, and combining the coherent optical waves, various types of optical interferometers have been implemented into optical interferometric systems including the Fabry-Perot interferometer (FPI), Fizeau interferometer, Michelson interferometer (MI), Mach-Zehnder interferometer (MZI) and Sagnac interferometer. These interferometers have found a wide variety of applications in various scientific and engineering fields.
Although optical interference and optical interferometers have many uses, they have also shown certain limitations such as the limited dynamic range, high-cost of implementation, stringent requirements on surface qualify and fabrication precision, difficulty to be multiplexed, and strong dependence on the material and geometry of the optical waveguides. As a result, optical interferometers have limited field applications despite their wide usage in laboratory conditions and controlled environments.
Microwave interferometers alleviate some of the limitations of optical interferometers. For example, construction of a microwave interferometer does not necessarily require a manufacturing accuracy as high as that in an optical interferometer. In addition, the stringent requirements on optical waveguides (e.g., geometry, dispersion, modes, and material characteristics) for making an optical interferometer can be relieved significantly in microwave interferometers.
However, microwaves cannot transmit over a long distance in a waveguide because of the large dielectric loss of the medium used for construction of the waveguide. Meanwhile, microwave waveguides are usually large in size (e.g., the most commonly used coaxial cable has a typical diameter on the order of several millimeters). In addition, pure microwave interferometers are susceptible to electromagnetic interference (EMI). As a result, pure microwave interferometers also have limited applications, especially when used as a sensor.